COMPASS AND PLUMB-BOB METHODS 141 



position the whole of the recording elements (plumb bob 

 and compass) remain until the point to be surveyed in the 

 hole is reached. In due course a pin y engages with the 

 end of lever u (Fig. 1) until finally cam q engages pin z in 

 the position shown in Fig. 4. This is the position of the 

 clamping device after the apparatus has arrived at the 

 survey point in the hole and sufficient time has elapsed for 

 the compass box to come to rest. Here the compass box is 

 clamped with needle and plumb bob free on their pivots. 

 Further movement of lever u disengages a projection or 

 tooth giving the position shown in Fig. 6. The ensuing 

 movement of cam q actuates levers clamping the plumb bob 

 e and, simultaneously by levers and connecting rods, releas- 

 ing the compass box and clamping the needle by spring x 

 as already said. It is clamped in the magnetic north so 

 that the free compass box gives us the direction of deviation 

 of the borehole directly; and the graduations on the plumb 

 bob relative to the zero indicator on the cover plate give the 

 amount of dip, also directly. 



It will thus be seen that this instrument enables the data 

 to be clamped or released after insertion and also gives 

 control over the recording elements. The clock can be set 

 at will independent of its mainspring and both compass 

 and plumb bob can also be reset at will. All readings are 

 direct with no additional surface computations and each 

 element is fully controlled. 



Objections which can be raised against this apparatus are 



a. The mechanism is too complicated and refined for 

 small holes. 



h. Readings are not continuous down the hole, each 

 point requiring extraction and reading at surface. 



c. Liability to mechanical complications. 



d. Compass unreliable in magnetic strata and lined holes. 



The Briggs Clinophone. — This is a plumb bob or pendu- 

 lum device with aural electrical registration applying the 

 Wheatstone bridge principle and is employed in precision 

 surveys of boreholes. That is to say, it is used where 

 deviations of more than 1 in 150 are not permissible, as in 



